Chapter 6 Section 6 2 6 3 6 2 Enthalpy Heats of Reaction and Chemical Change The Meaning of Enthalpy To determine E we must measure both heat and work The two most important types of chemical work are electrical work i e transferring electrons and pressure volume P V work i e the work done by an expanding gas w P V For reactions at constant pressure we define another variable called enthalpy Enthalpy H is defined as the internal energy E plus the product of pressure times volume H E PV The change in enthalpy H is equal to the change in energy E plus the product of constant pressure times the change in volume V H E P V Recall E q w H q w P V w P V H q P V P V H q Exothermic and Endothermic Processes Because E P and V are state functions H is also a state function which means H Hfinal Hinitial Hproducts Hreactants exothermic process releases heat H 0 H 0 Hinitial heat out CO2 2H2O A H2O g H 0 H2O l Hfinal Exothermic process Think Heat is a PRODUCT Enthalpy H Enthalpy H CH4 2O2 endothermic process absorbs heat H 0 B Hfinal heat in Hinitial Endothermic process Think Heat is a REACTANT Select the endothermic reaction s from the below A H2 g O2 g H2O l 258 8 kJ B 40 7 kJ H2O l H2O g C C3H5 NO3 3 l 3CO2 g 5 2 H2O g O2 g 3 2 N2 g 5720kJ D 178 3 kJ CaCO3 s CaO s CO2 g E Both answers B and D are endothermic reactions In an endothermic reaction heat is gained by the system that is heat acts as a reactant 6 3 Calorimetry Measuring the Heat of a Chemical or Physical Change Specific Heat Capacity The quantity of heat absorbed by an object is proportional to its temperature change q T OR q constant x T OR constant q T Every object substance has its own heat capacity that is the quantity of heat required to change its temperature by 1 K and is equal to q T Heat capacity q T in units of J K The specific heat capacity c is the quantity of heat required to change the temperature of 1 gram of a substance by 1 K Specific heat capacity c q mass x T Units J gK If we know the specific heat capacity of a substance we can measure its mass and temperature change to determine heat absorbed or released q c x mass x T If a substance gets hotter T Tfinal Tinitial is positive and q 0 If a substance cools T is negative and q 0 The molar heat capacity C is the quantity of heat required to change the temperature of 1 mole of a substance by 1 K Molar heat capacity C q mol x T Units J mol K Sample Problem 6 3 A layer of copper welded to the bottom of a skillet weighs 125 g How much heat is needed to raise the temperature of the copper layer from 25 C to 300 C The specific heat capacity c of copper is 0 387 J g K q c x mass x T T in C 300 C 25 C 275 C T in K 300 C 273 15 25 C 273 15 573 15 K 298 15 K 275 K q c x mass x T T is the same in either Celsius or Kelvin 0 387 J g K x 125 g x 275 K 1 33 x 104 J Find the heat transferred in J when 6105 g of ethylene glycol in a car radiator cools from 37 0 C to 25 0 C c 2 42 J g K A 1 40 x 107 J B 1 77 x 105 J C 3 69 x 105 J Mistake Multiplied by 25 C rather than T D 5 47 x 105 J Mistake Multiplied by 37 C rather than T E 9 16 x 105 J Mistake Multiplied by T rather than T q c x mass x T 2 42 J g K x 6105 g x 12 0 K 177 289 J 1 77 x 105 J T in C 25 0 C 37 0 C 12 0 C 12 0 K The Practice of Calorimetry A calorimeter is a device used to measure the heat released or absorbed by a physical or chemical process The calorimeter is the surroundings that changes temperature when heat is transferred to or from the system Two common types are constant pressure and constant volume calorimeters Constant Pressure Calorimetry A coffee cup calorimeter Figure 6 7 is often used to measure the heat transferred in processes open to the atmosphere One common use is to find the specific heat capacity of a solid that does not react with or dissolve in water 1 The solid system is weighed and heated to a known temperature Tinitial solid 3 After stirring the water for some time the temperature of the water stops changing Tfinal solid H2O Water Temperature C 2 The hot solid is added to a sample of water at a known temperature Tinitial H2O and mass x x x x x x x Tf solid H2O Twater TfH2O TiH2O Tsolid Tf solid Ti solid xx x add hot solid Ti H2O Time minutes Constant Pressure Calorimetry Heat released by system solid is equal in magnitude and opposite in sign to heat absorbed by surroundings water qsolid qH2O Recall q c x mass x T csolid x masssolid x Tsolid cH2O x massH2O x TH2O csolid cH2O x massH2O x TH2O masssolid x Tsolid Sample Problem 6 4 Determining the Specific Heat Capacity of a Solid PROBLEM A 22 05 g solid is heated in a test tube to 100 00 C and added to 50 00 g of water in a coffee cup calorimeter The water temperature changes from 25 10 C to 28 49 C Find the specific heat capacity of the solid Pull out what we know Masssolid 25 64 g MassH2O 50 00 g Tinitial solid 100 00 C Tfinal solid 28 49 C Tinitial H2O 25 10 C Tfinal solid 28 49 C csolid cH2O 4 184 J gK Relevant equations csolid x masssolid x Tsolid cH2O x massH2O x TH2O Sample Problem 6 4 SOLUTION csolid x masssolid x Tsolid cH2O x massH2O x TH2O Twater Tfinal Tinitial 28 49 C 25 10 C 3 39 C 3 39 K Tsolid Tfinal Tinitial 28 49 C 100 00 C 71 51 C 71 51 K csolid cH2O x massH2O x TH2O masssolid x Tsolid 4 184 J g K x 50 00 g x 3 39 K 22 05 g x 71 51 K 0 450 J g K Select the phrase that will make the following statement true In a constant pressure calorimetry experiment A an increase in the recorded temperature indicates heat is transferred from the system solid to the surroundings water B a decrease in the recorded temperature …